2022 Fiscal Year Final Research Report

Clarification of microstructural formation process in displacive transformation of steels by three-dimensional microstructural analysis

Research Project

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Project/Area Number	20H02475
Research Category	Grant-in-Aid for Scientific Research (B)
Allocation Type	Single-year Grants
Section	一般
Review Section	Basic Section 26050:Material processing and microstructure control-related
Research Institution	The University of Tokyo
Principal Investigator	Nambu Shoichi 東京大学, 大学院工学系研究科(工学部), 准教授 (00529654)
Project Period (FY)	2020-04-01 – 2023-03-31
Keywords	鉄鋼材料 / マルテンサイト変態 / 3次元組織観察 / 結晶方位関係
Outline of Final Research Achievements	The purpose of this study was to 3-dimensionally understand the microstructural formation process during displacive transformation, in the early stage of transformation, to control the microstructure to achieve further improvement of next-generation-high-strength steels. By combining optical microscopy based serial sectioning and crystal orientation analysis by SEM/EBSD, we obtained 3-dimensional microstructures in a larger area, and made it possible to investigate the relationship between the microstructure formed in the early stage of transformation and the prior austenite grain boundaries. In the upper bainite, bainite pairs with a small misorientation are formed on the both sides across the grain boundary, which indicates that the crystal orientation of adjacent austenite affects the bainite formation. Lower bainite is formed along the grain boundary plane, which indicates that the misorientation between the grain boundary plane and the habit plane affects the formation behavior.
Free Research Field	組織制御
Academic Significance and Societal Importance of the Research Achievements	次世代高強度鋼では強度と延性の高いレベルでの両立が求められており，そのためにはせん断型変態で得られるベイナイトやマルテンサイトと残留オーステナイトで構成される複相鋼の最適な組織制御が必須である。これらの組織を3次元的に最適化するためには，変態初期から中期にかけての組織制御が重要であり，本研究で確立した3次元組織評価手法によって，変態初期にオーステナイト粒界から生成する組織の3次元的な形態，生成サイト，結晶方位選択等を理解することができ，これら成果はせん断型変態機構の解明や組織制御指針の構築に向けて学術的にも工学的にも非常に有益であると言える。